(1) Field of the Invention
The present invention relates generally to non-destructive testing. More particularly, the present invention relates to a sensor array system for structural monitoring in which the sensors are arranged in a continuous series connection for increasing the likelihood of detecting a critical event.
(2) Description of the Prior Art
The performance of modem-day military helicopters, missiles, tanks, aircraft, and other static or dynamic structures is critically dependent on the reliability of advanced composite materials and heterogeneous armor materials. There has been a reluctance to deploy such high performance materials in critical structural applications because of their susceptibility to in-service damage. The damage occurring in these materials may be difficult to track and can propagate quickly during operation of the vehicle or structure, resulting in the loss of the entire vehicle.
Conventional non-destructive evaluation techniques are labor intensive, expensive, error prone, and unworkable for efficient integration into composite and heterogeneous structures. Autonomous integrated Structural Health Monitoring (SHM) techniques are a revolutionary concept in the maintenance of structures. SHM techniques continuously monitor the condition of a stricture. Various approaches for SHM under development use piezoceramic sensors and actuators that require separate wiring connections for each sensor and actuator element, storage of pre-damage data for each sensor, and instrumentation for active generation and sensing of diagnostic signals. When the structural geometry is complexxe2x80x94e.g., either the structure has varying thickness, curvature, ribs, joints, or heterogeneous materials, or damage is located near boundaries of the structurexe2x80x94it becomes difficult to detect small damage using SHM methods. In addition, the number of sensor circuits and computations required increases the overall complexity and cost of the structure.
One approach to this problem is to integrate many fiber-optic strain gauges directly within the structural material. An optical fiber with twenty or more Bragg gratings can measure static and dynamic strains at discrete locations on the structure. An optical analyzer can multiplex over each fiber and each grating to measure strains at a large number of points on a structure. This approach is being implemented on bridges, pressure tanks and other structures. However, fiber optic sensors have limitations when applied to monitoring complex composite structures where damage can occur anywhere on the structure and in any direction. For example, discrete strain measurements can miss damage because the measurement is very localized at the fiber/grating. In addition, an optical analyzer using multiplexing and multiple connections is expensive; measurements are not simultaneous and the frequency bandwidth may be too low to sense Acoustic Emission (AE) signals.
AE sensors are presently suitable for detection of damage at xe2x80x9chot spots. xe2x80x9d The use of AE measurements for SHM of large structures may have certain advantages since it is a passive sensing technique. Passive sensing methods are simpler and may be more practical than using active interrogation methods. However, present passive acoustic emission and monitoring techniques require bulky instrumentation with numerous channels, long connections, and centralized data analysis. It may be impractical to embed these systems on the structure to operate in the field. Another limitation is that AE waveforms from such sensors are too complicated for purposes of source characterization.
Thus, there remains a need for a new and improved sensor array system for structural health monitoring that will provide sufficient spatial coverage to efficiently sense AE signals and, at the same time, is simple and cost effective.
The present invention is directed to a sensor array for non destructively monitoring a structure to detect a critical event. The sensor array includes a plurality of discrete sensor nodes, each of which produces an electrical signal in response to a structural event. The sensor nodes may include either a piezoceramic (PZT) wafer, a plurality of piezoceramic fibers, accelerometers, or chemical sensors arranged in either a planar or three dimensional array. In the preferred embodiment, the sensor nodes include a plurality of piezoceramic fibers. The fibers are aligned substantially parallel to each other within the sensor node and are electroded and poled to act as a single sensor node. The fibers have a plurality of polarized regions that are substantially oriented in series according to their polarity in a +xe2x88x92+xe2x88x92+xe2x88x92 . . . arrangement, or, alternatively, in parallel with each other. An adding circuit connects the plurality of polarized regions in series with one another. A signal adder receives and combines the electrical signals from each of the discrete sensor nodes to form a single sensor array output signal.
In the preferred embodiment, the electrical interface further includes at least one electrical bus substantially aligned with the fibers and a signal processing module for receiving and processing the single sensor output signal. The signal processing module includes an input connected to the signal adder, an impedance matching amplifier connected to the input for amplifying the single sensor output signal, and an output. The signal processing module may further include an electronic noise filter.
In addition, the sensor array may further include an electronic trigger for detecting the beginning of an event and providing a trigger signal and a display, such as an oscilloscope, connected to the trigger and the output, whereby the trigger signal activates the display for observing the output.
The sensor array also may further include a threshold detector for detecting an output exceeding a predetermined threshold level and providing an alarm signal when the output exceeds the predetermined threshold level.
The plurality of discrete sensor nodes may further be divided into discrete subgroups, termed unit cells, each located at a different structural location. For example, a subgroup could be part of each rotor blade of a helicopter or different armor panels of a tank to provide a degree of sensing the location of the structural event in a specific element of the structure.
In the preferred embodiment, the discrete sensor nodes in one or more subgroups are electrically connected in series, thereby forming a continuous series connection between each of the discrete sensor nodes to improve the likelihood that a critical structural event will be detected.
Accordingly, one aspect of the present invention is to provide a sensor array for nondestructively monitoring a structure to detect a critical event. The sensor array includes a plurality of discrete sensor nodes in which each of the discrete sensor nodes produces an electrical signal in response to a structural event, and a signal adder for receiving and combining the electrical signals from each of the discrete sensor nodes to form a single sensor array output signal.
Another aspect of the present invention is to provide a sensor for nondestructively monitoring a critical event. The sensor includes: at least one sensing node having a plurality of piezoceramic fibers arranged in a planar array in which the fibers are aligned substantially parallel to each other within the sensing node, wherein each of the fibers has a plurality of polarized regions that are substantially oriented in series according to their polarity in a predetermined arrangement; and an electrical interface connecting the plurality of polarized regions of each sensor node in series with one another.
Still another aspect of the present invention is to provide a sensor array for nondestructively monitoring a structure to detect a critical event. The sensor array includes: a plurality of discrete sensor nodes in which each of the discrete sensor nodes produces an electrical signal in response to a structural event, wherein the sensor nodes include a plurality of piezoceramic fibers arranged in a planar array in which the fibers are aligned substantially parallel to each other, each of the fibers has a plurality of polarized regions that are substantially oriented according to their polarity in a predetermined arrangement; an electrical interface connecting the plurality of polarized regions of each fiber in series with one another; and a signal adder for receiving and combining the electrical signals from each of the discrete sensor nodes to form a single sensor array output signal.
These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings.